Monitoring circuits for lamps or other electrical loads using a comparator with a constant voltage power supply

ABSTRACT

A circuit for monitoring electrical loads, e.g. lamps especially in motor vehicles, comprises a power supply line (14) for supplying electric current for illuminating a lamp (10, 12), and means (58) for providing a reduced electric current through the lamp when not illuminated. A resistor (24) is in series with the lamp, and a comparator (40) is arranged to compare the potential of a point between the resistor and the lamp with a reference potential when the lamp is not illuminated for generating a warning signal in the event of lamp failure. A further power supply line (48) is also provided, and means (50) for establishing a fixed potential difference between the further and the first power supply lines. The comparator is arranged such that the fixed potential difference provides a constant voltage power supply for the comparator.

TECHNICAL FIELD

This invention relates to electrical monitoring circuits for detectingthe failure of electrical loads, and is especially applicable tomonitoring the serviceability of lamps, e.g. incandescent filament lampsor light emitting diodes, and can be used for monitoring the lamps ofmotor vechicle lighting systems.

BACKGROUND ART

Electrical monitoring circuits are known which provide a warning signal,usually in the form of a warning light, indicating the failure of or afault in a lamp. One such system, described in U.K. Patent No.1,342,248, monitors the lighting circuits of a motor vehicle in order todetect any lamp failures which may occur. In that system the potentialdifference across a low value resistor inserted in series with each lampis monitored and when the potential difference is zero a fault isindicated.

Another such system, also employing a low value series resistor as adetector and designed to overcome undesirable temperature-dependencecharacteristics of earlier systems, is described in U.K. Patent No.2,034,948 B. Moreover, this system provides additional monitoringfacilities to discriminate between lamp failure and faults due to thelow value series resistor being open-circuit or a fuse failure. Thislatter patent also describes in detail various integrated circuitembodiments of such systems which can monitor groups of lamps includinglamps which are required to operate only intermittently such as thevechicle's brake lights.

The known systems provide an indication of the failure of a vehicle'slamps during their use only and may be described as energised or `hot`monitoring systems; they are not capable of monitoring a lamp that isnot energised, i.e. they do not have a de-energised or `cold` monitoringcapability.

It is an object of the present invention to provide a means whereby thedetection of a failure of the filament of a vehicle's lamp may be madewhilst that lamp is de-energised or `cold`.

It has been suggested that `cold` monitoring could be achieved byproviding a bleed resistor across the on/off switch for the lamps, sucha resistor providing a small leak current through the lamp(s) andassociated in-series resistors when the switch is open; the voltageacross the in-series resistor could then be monitored in the same way as`hot` monitoring. However, the leak current must be very low so as toavoid undue draining of the battery and thus the voltage drop across thein-series resistor would be minute (e.g. 10 microvolts, which contrastswith a typical voltage drop during hot monitoring of approximately 20millivolts).

A technique known as `auto-zeroing` has been known for several year toreduce amplifier (and comparator) errors to very low (unmeasurable)levels. This technique uses two operational amplifiers which alternatein serving the input signals; while one amplifier is performing thecomparison (or amplification) function, the other is nulling itself andstoring a correction term on a capacitor. The input is then transferredto the recently nulled amplifier and the previous amplifier corrects itserrors. This cycle is repeated under a suitable controlling switchingcircuit. No-one has suggested applying such a technique to coldmonitoring of vehicle bulbs but it is not a technique which is readilyapplicable to use in vehicles since auto-zeroing is highly sensitive tovoltage transients and so would be unreliable in a vehicle where thevoltage supplied by the battery can vary considerably depending on theload on the battery.

A further problem arises if the same comparator is to be used to performboth `cold` monitoring and `hot` monitoring since the input to thecomparator in the former case would be a potential close to earth whilethe input in the latter case would be the voltage rating of the battery(usually 12 volts) and this would lead to great unreliability inoperation of such a sensitive comparator.

DISCLOSURE OF THE INVENTION

Circuits have now been developed for monitoring the serviceability ofelectrical loads (and especially lamps) that are capable of passing arelatively small current when the load is not operational but that dooperate when a relatively larger current is passed through them. Thecircuit finds special application in monitoring loads powered bysupplies, e.g. batteries, whose potential can fluctuate.

According to the present invention, there is provided a circuit formonitoring the serviceability of at least one electrical load, e.g. alamp, when it is not energised, which circuit comprises a first powersupply rail for supplying electric current to the load, a resistor inseries with the load, means for providing a bleed current through theload and the resistor when the load is unenergised (which current isusually less than 100 mA, e.g. less than 50 mA, and preferably about 5to 20 mA for a 12 volt power supply), a comparator capable of comparingthe potential of a point between the resistor and the load when the loadis not energised with a reference potential, a further power supplyrail, and means for establishing a fixed potential difference betweenthe first power supply rail and the further power supply rail andwherein the said comparator has as a power supply the fixed potentialdifference between the first power supply rail and the further powersupply rail.

The comparator is preferably such that it can detect potentialdifferences of less than 50 microvolts between its inputs, andpreferably less than 20 microvolts. It is preferably an auto-zeroingcomparator.

Using such an arrangement, the comparator is driven from a constantpotential power supply irrespective of the voltage supplied to the loadand so gives a consistent and reliable output; this is particularlyimportant in the case of a vehicle battery the potential supplied bywhich can vary depending on the amount of current drawn by other vehicleequipment.

In a preferred form of the invention, the comparator has a floatingpower supply in that the first power supply rail is connected to thepower supply, e.g. the vehicle battery, by a switch, and the arrangementis such that, when the first power supply rail is disconnected from thepower supply by the switch, the comparator compares potentials close toearth potential and one terminal of its own power supply is close toearth potential, whereas when the loads are energised (i.e. operational)and the first power supply rail assumes a higher potential (usually 12volts), the comparator compares signals close to 12 volts which, again,is a potential close to the potential of one terminal of the comparatorpower supply. Thus the same comparator can be used for both `hot` and`cold` monitoring.

According to a second aspect of the present invention there is provideda circuit for monitoring the serviceability of at least one load when itis not energised (cold monitoring) and when it is energised (hotmonitoring), which circuit comprises a first power supply rail, a switchfor connecting the first power supply rail to a source of potential toenergise the load, a resistor in series with the load, means forproviding a bleed current to the load and the resistor when the switchis open, a second power supply rail, means for establishing a fixedpotential difference between the first power supply rail and the secondpower supply rail, a comparator for comparing first and second signalssupplied to first and second inputs thereof and providing an outputsignal indicative of failure of the load when the signals supplied tothe first and second inputs have a predetermined relationship to eachother, wherein the power supply of the comparator is derived from thefixed potential difference between the first and the second supplyrails, and wherein the first comparator input signal is a signalindicative of the potential of a point between the resistor and the loadand the second comparator input signal is a reference signal indicativeof a first reference potential during cold monitoring and a secondreference potential during hot monitoring.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in further detail, by way ofexample only, with reference to the accompanying circuit diagrams inwhich:

FIG. 1 shows a circuit according to the present invention which iscapable of monitoring the serviceability of lamps both when they areilluminated (hot monitoring) and when they are not (cold monitoring),and

FIG. 2 illustrates the operation of part of the circuit of FIG. 1 knownas the charge pump.

BEST MODE FOR CARRYING OUT THE INVENTION

The `hot` and `cold` monitoring circuit of FIG. 1 includes a first and asecond bulb 10 and 12, e.g. the left and right brake lights of avehicle, which are connected between a power supply rail 14 and an earthline 16. The power rail 14 can be energised to switch on the bulbs byconnecting it to the main power line 18 of the vehicle by means of aswitch 20; the main power line is connected to the vehicle battery (notshown) by the ignition switch 22. Thus, when the ignition is switched onand switch 20 is closed, current is supplied to rail 14 and so energises(or lights) lamps 10 and 12.

The lamps 10 and 12 are each connected in series with a low valueresistor 24, typically having a rating of 1.25 milliohms. The potentialsv₁ and v₂ at points 23 and 25 between resistors 24 and their respectivelamps 10 and 12 are fed by leads 26, 28 to channels 1 and 2 of amultiplexer 30 (RCA 4053).

Hot Monitoring

Connected between power supply line 14 and the earth line are a firstresistor 32 (about 0.024 ohms), two further resistors 34 and 35 (24K and120 ohms respectively) and a known integrated circuit 36 forming one arm(the `hot` monitoring arm 31) of a wheatstone bridge, the other arm ofwhich is formed by one of the lines containing bulbs 10 and 12 as willbecome evident later; the potential v₃ at a point 37 between the tworesistors 32 and 34 is fed by lead 38 to pin 5 of the multiplexer 30.The integrated circuit 36, together with the resistors 34 and 35,emulates the potential drop across bulbs 10, 12 when energised. It wouldnot be possible to emulate the potential drop across lamps with a singleresistor because the resistance of the lamp filament varies markedlywith the applied voltage due to the change in resistance of the filamentwith a change in its operating temperature which is brought about by achange in the potential across the filament. Integrated circuit 36 andresistor 35 form a constant current source of known design and the sumof the fixed current through circuit 36 and the variable current throughresistor 34 emulates the behavior of lamps 10, 12 with changing voltage.The values of the resistors 24 and 32 and the resistance of thecomponents 34 to 36 are chosen to balance the wheatstone bridge when thebulbs 10 and 12 are intact. The resistance provided by the `hot`reference arm should be relatively high (compared to the combinedresistance of bulb 10 or 12 and one of the resistors 24) to minimise thecurrent flowing through the `hot` reference arm.

An auto-zeroing comparator 40 (ICL 7652/LTC 1052 CN8) is provided forcomparing the potential v₃ with potentials v₁ and v₂, the potentials v₁and v₂ being alternately supplied to the comparator for comparison withpotential v₃ by the multiplexer 30 as will be described in greaterdetail below. Auto-zeroing comparators are known integrated circuits andcontain two operational amplifiers which alternate in comparing thepotential on two input lines 42 and 44 connected to output pins 15 and 4of the multiplexer 30. While one amplifier is performing the comparisonfunction the other is nulling itself and storing its correction term onone of two capacitors, C₂, C₃ (0.1 microfarad each). When this has beenperformed, the inputs are transferred to the second amplifier while thefirst amplifier corrects itself and stores its correction term on theother of the capacitors C₂, C₃. Such a comparator can distinguishextremely small voltage differences between its inputs, e.g. 10microvolts, and such a comparator could not normally be used in avehicle system because it would be sensitive to changes in the supplyvoltage at its power input, e.g. caused by changing temperature or by achanging load on the battery, but it can be used in the arrangement ofthe present invention because power is supplied to the comparator 40through a further power supply rail 48, which is maintained at a fixedpotential relative to the supply rail 14 by means of a circuit 50 whichis a charge pump of known design (see LTC 1044.7660 Switched BatteryVoltage Converter Texas Instruments (May 1987)). The charge pump 50maintains the potential of rail 48 at a fixed value, e.g. at +5 V,relative to the potential at point C on rail 14. Since the power for thecharge pump 50 is derived from line 18 between the ignition switch 22and the lamp switch 20, it operates only when the ignition is switchedon. Then, when switch 20 is open and point C is at approximately groundpotential, the charge pump maintains rail 48 at +5 V. However, whenswitch 20 is closed to energise the lamps, the potential of point Cincreases to +12 volts and so the potential of rail 48 is increased to+17 volts.

The operation of the charge pump is as follows (with reference to FIGS.1 and 2): a voltage of 5.6 V set by zener diode Z₁ is switched to chargecapacitor C₄ by oscillating switch S₁ which is integrated in the circuit54 shown in FIG. 1 and which is switched rapidly between its twopositions by an oscillator also within integrated circuit 54. Thecapacitor C₄ is thus charged to a potential of 5.6 volts and thispotential is applied as an input to a summing junction J. Dependingwhether the lamp switch 20 is closed or open, a potential of either 12volts or 0 volts (respectively) will also be applied as a further inputto the summing junction J from point C by way of a diode 53. The outputfrom the summing junction J is applied by way of a diode 55 to thecapacitor C₁. If the current drawn by the overall load between the firstand second power supply rails 14 and 48 is small compared to the currentsupplied to the second supply rail 48 and to the capacitor C₁ (as is thecase with the current drawn by the comparator 40), then the capacitor C₁will accumulate the voltage across the zenner diode Z₁. In fact, due tothe voltage drop across diode 55 of approximately 600 mV, the voltageacross capacitor C₁ is approximately 5 V. i.e. the second rail 48 ismaintained at a potential of +5 V as compared to first rail 14. Sincethe power terminals of comparator 40 are connected to rails 14 and 48,the comparator is always supplied with a fixed voltage set by the chargepump 50, e.g. 5 V, irrespective of the potential of line 14.

Returning to FIG. 1, if the bulbs 10, 12, are intact, the potentials v₁and v₂ are equal to the potential v₃ and the comparator will provide azero output. If, however, the lamp 10 fails, the potential at v₁increases to the potential of line 14 and this imbalance will causecomparator 40 to provide an output which makes transistor T3 conductingwhich in turn makes transistor T4 conducting and so energises LED 56giving a warning of this lamp failure. The power for energising LED 56is derived from the ignition circuit 18 of the vehicle.

Cold Monitoring

Cold monitoring is achieved by allowing a small bleed current to flowthrough resistor 58 (rating 1 kohm divided by the number of bulbs beingmonitored by the channel, i.e. with two bulbs as show, resistor 58 wouldhave a resistance of 500 ohms). In the circuit given, this produces acurrent of about 12 mA through each resistor 24 and through the bulbs 10and 12 causing a voltage drop of about 15 microvolts across theresistors when the bulbs are intact. The voltages v₁ and v₂ are fed toinputs 1 and 2 of multiplexer 30 and are alternately supplied tocomparator input 42 and so compared against a reference potential v₄derived from a point 52 between resistors 60 and 62 (10 ohms and 24kohms respectively) in a `cold` monitoring arm 59. Resistors 60 and 62are of such values that potential v₄ equals potentials v₁ and v₂ whenthe bulbs 10 and 12 are intact. Compared to the combined resistance ofresistor 24 and bulb 10 or resistor 24 and bulb 12, the resistance of`cold` monitoring reference arm 59 (containing resistors 60 and 62) ishigh so that the amount of current drawn down arm 59 is very low.

In cold monitoring, there is no need to include an integrated circuit 36since the resistance of bulbs 10 and 12 are practically constant.

The bleed of 24 mA (in the case of two bulbs per channel) from thebattery will not produce a significant drain on the vehicle batteryespecially since the monitoring system is switched off by the ignitionswitch 22. The value of the resistor 58 should be chosen so that thevoltage drop across resistors 24 is high enough to be measurable by thecomparator without drawing excessive current from the battery.

Operation

During `cold` monitoring, the potential v₄ is supplied by multiplexer 30to the input line 44 of the comparator while during `hot` monitoring, itis the potential v₃ that is supplied by multiplexer 30 to the input line44. The multiplexer 30 also supplies the voltages v₁ and v₂ alternatelyto the other input line 42 of comparator. Thus during `cold` monitoring,a wheatstone bridge is formed with one arm thereof being the `cold`monitoring arm 59 and the other arm being alternately formed by the linecontaining lamp 10 and the line containing the lamp 12. During `hot`monitoring the first arm of the bridge is instead formed by `hot`reference arm 31.

Switching of the multiplexer 30 is conducted as follows: when switch 20is open and ignition switch 22 is closed, the voltage on line 14 isabout 29 mV above earth potential of ground rail 16. This causestransistor T2 to be non-conductive so that the voltage applied to inputs9 and 16 of the multiplexer are equal (about 5 volts); under thesecircumstances, the multiplexer connects its input pin 3 (providingpotential v₄) to its output pin 4 and so the voltage v₄ of coldmonitoring bridge arm 59 is fed to the comparator input 44. When switch20 is closed, causing bulbs 10 and 12 to become energised and requiringhot monitoring, the voltage on rail 14 rises to 12 volts causingtransistor T2 to become conducting; this results in different voltagesbeing applied to pins 9 and 16 of multiplexer 30, causing multiplexer toconnect its input pin 5 (supplying the voltage v₃ of hot monitoringbridge arm 31) to output pin 4 of the multiplexer and hence to inputline 44 of the comparator 40.

The multiplexer 30 is also switched to alternately connect potential v₁and potential v₂ to output pin 15 of multiplexer 30 and hence to inputline 42 of comparator 40; this is achieved by applying a zero potentialand a positive potential alternately at point 66. When the potential iszero, transistor T1 is non-conductive and the potential applied to pin10 of the multiplexer is the potential of rail 48 (5 or 17 volts)causing the signal applied to pin 1 of the multiplexer (i.e. voltage v₁)to be applied to input lead 42 of the comparator 40. When a potential isapplied at point 66, the transistor T1 is rendered conductive causingthe potential of pin 10 of the multiplexer to drop to the potential ofrail 14, thereby causing the multiplexer to connect its input pin 2,i.e. potential v₂, to comparator input line 42.

It is possible to include more than one bulb in each arm (as shown byghost lamps 70 and 72). In this case, the value of the bleed resistor 58and of the resistors 24 should be such that the voltage drop acrossresistor 24 when both of the parallel bulbs 10, 70 or 12, 72, are intactis approximately the same as the voltage drop across the cold referenceresistor 60 and hot reference resistor 32 but when one or both of theparallel bulbs has failed the voltage drop across resistor 24 drops(i.e. the potential v₁ or v₂ rises) causing an imbalance at comparator40 and the triggering of the LED 56 as discussed above.

As will be appreciated the number of lines containing lamps can begreater than two, in which case the control for switching themultiplexer 30 will connect each line in turn to the comparator 40.

INDUSTRIAL APPLICABILITY

Although the invention has been described in connection with themonitoring of incandescent lamps, the present invention can be appliedin the monitoring of any electrical load that is capable of passing ableed current when it is not operational and is particularly applicablein monitoring loads powered by a fluctuating potential power source. Thecircuit can be used in fields other than vehicles, e.g. in aircraft (formonitoring both the external lamps and also the instrument controllamps), in ships and boats, in telecommunications, in industrial andautomation control, and in medical and environmental control.

The circuit shown in FIG. 1 can be incorporated into a simple integratedcircuit.

I claim:
 1. A circuit for monitoring the serviceability of electricalloads comprising a power supply line (14) for supplying electric currentto the load (10, 12) to energise it, means (58) for providing a reducedelectric current through the load when not energised, a resistor (24) inseries with the load, and a comparator (40) arranged to compare thepotential of a point between the resistor and the load with a referencepotential when the load is so connected that it is not energised forgenerating a warning signal in the event of load failure, andcharacterised by a further power supply line (48), and means (50) forestablishing a fixed potential difference between the further and thefirst power supply lines, the comparator being arranged such that thefixed potential difference between the further and the first supplylines provides a constant voltage power supply for the comparator.
 2. Amonitoring circuit according to claim 1, characterised in that thereduced current is a bleed current, which is less than 100 mA.
 3. Amonitoring circuit according to claim 2, characterised in that the bleedcurrent is approximately from 5 mA to 20 mA.
 4. A monitoring circuitaccording to claim 1, characterised in that the comparator is capable ofdetecting potential differences of less than 50 microvolts between thepotential of the point between the resistor and the load and thereference potential.
 5. A monitoring circuit according to claim 1,characterised in that the comparator is capable of detecting potentialdifferences of less than 20 microvolts between the potential of thepoint between the resistor and the load and the reference potential. 6.A monitoring circuit according to claim 4, characterised in that thecomparator is an auto-zeroing comparator.
 7. A monitoring circuitaccording to claim 1, characterised in that the first power supply linehas a potential level determined according to an output from a parallelconnection of a switch (20) and the reduced current providing means, andin that the fixed potential establishing means comprise a charge pumparranged to control the potential level of the further power supply linein dependence upon the potential level of the first power supply line.8. A monitoring circuit according to claim 1, characterised in that thecomparator is arranged to compare the potential of the point between theresistor and the load with a second reference potential when the load isconnected to be energised for generating a warning signal in the eventof load failure.
 9. A monitoring circuit according to claim 1,characterised by including a plurality of the loads, a respectiveresistor in series with each load, and means (30) for successivelyconnecting the points between each resistor and the associated load tothe comparator.
 10. A load monitoring circuit comprising a power supplyline (14) for supplying electric current to a load (10, 12), a resistor(24) in series with the load, and a comparator (40) arranged to comparethe potential of a point between the resistor and the load with a firstreference potential when the load is so connected that it is notoperational and with a second reference potential when the load isconnected to be operational for generating a warning signal in the eventof load failure, characterised in that the comparator is connectedbetween a further power supply line (48) and the first power supplyline, and in that means (50) are provided for establishing a fixedpotential difference between the further and the first power supplylines.
 11. A monitoring circuit according to claim 1, characterised bythe load being a lamp.
 12. A load monitoring circuit comprising a load,a switch for switching said load on and off, means for supplying a firstelectric current to said load when said switch is on and a reducedelectric current to said load when said switch is off, said means forsupplying including a first power supply line, a resistor connected inseries with said load and defining with said load a potential monitoringpoint, means for providing a reference potential having approximatelythe same magnitude as the potential produced at said potentialmonitoring point when said reduced electric current is supplied to saidload and said load is functioning normally, a comparator arranged tocompare the potential produced at said potential monitoring point withsaid reference potential when said reduced electric current is suppliedto said load and to generate a signal in the event that said load is notfunctioning normally, a further power supply line, and means forestablishing a fixed potential difference between said further and saidfirst power supply lines, said comparator being coupled between saidfurther and said first power supply lines whereby said fixed potentialdifference provides a constant voltage power supply for the comparator.13. A load monitoring circuit comprising a load, a power supply line forsupplying current to said load, a switch for switching said load on andoff, a resistor connected in series with said load and defining withsaid load a potential monitoring point, a source of first and secondreference potentials, a further power supply line, means forestablishing a fixed potential difference between said further and saidfirst power supply lines, and a comparator connected between saidfurther and said first power supply lines whereby said fixed potentialdifference provides a constant voltage power supply for said comparator,said comparator being arranged to compare the potential produced at saidpotential monitoring point with said first reference potential when saidswitch is off and with said second reference potential when said switchis on for providing a signal in the event of failure of said load.
 14. Aload monitoring circuit comprising a load, a power supply, a first powersupply line, a switch for connecting said first power supply line tosaid power supply for energising said load, a resistor connected inseries with said load and having a potential monitoring point betweensaid resistor and said load, means for providing a bleed current to saidresistor and said load when said switch is open, a second power supplyline, means for establishing a fixed potential difference between saidfirst and said second power supply lines, a comparator having first andsecond inputs for receiving, respectively, a first signal representingthe potential at said potential monitoring point and a second signalrepresenting a first reference potential when said switch is open and asecond reference potential when said switch is closed, said comparatorbeing connected between said first and said second power supply linesfor comparing said first and said second signals and providing an outputsignal indicative of load failure when said first and said secondsignals have a predetermined relationship with each other.